Sudden expansion of a single brown bear maternal lineage across northern continental Eurasia after the last ice age: a general demographic model for mammals?

The brown bear has proved a useful model for studying Late Quaternary mammalian phylogeography. However, information is lacking from northern continental Eurasia, which constitutes a large part of the species' current distribution. We analysed mitochondrial DNA sequences (totalling 1943 bp) from 205 bears from northeast Europe and Russia in order to characterize the maternal phylogeography of bears in this region. We also estimated the formation times of the sampled brown bear lineages and those of its extinct relative, the cave bear. Four closely related haplogroups belonging to a single mitochondrial subclade were identified in northern continental Eurasia. Several haplotypes were found throughout the whole study area, while one haplogroup was restricted to Kamchatka. The haplotype network, estimated divergence times and various statistical tests indicated that bears in northern continental Eurasia recently underwent a sudden expansion, preceded by a severe bottleneck. This brown bear population was therefore most likely founded by a small number of bears that were restricted to a single refuge area during the last glacial maximum. This pattern has been described previously for other mammal species and as such may represent one general model for the phylogeography of Eurasian mammals. Bayesian divergence time estimates are presented for different brown and cave bear clades. Moreover, our results demonstrate the extent of substitution rate variation occurring throughout the phylogenetic tree, highlighting the need for appropriate calibration when estimating divergence times.

Morphological Differentiation of the Skull in Two Closely-related Mustelid Species (Carnivora: Mustelidae).

Alexei V. Abramov, Andrey Yu. Puzachenko, and Igor L. Tumanov (2016) A morphological differentiation (i.e., the amount of morphological space occupied) in two polecat species, Mustela putorius and M. eversmanii, has been studied. These closely related species are similar in the body size, the age of origin, and many aspects of their natural history. We have used cranial characters to estimate some parameters of morphological diversity, to compare 'morphological niche breadth' occupied by polecats in the morphological space and their overlap, assuming that variation in the characteristics of morphological diversity could be reflected in the extent of adaptive diversification. A comparison of diversity based on 23 cranial characters shows that the polecats occupied distinct areas of the morphospace. Both skull 'size' and 'shape' characters are important components of the morphological differentiation between M. putorius and M. eversmanii. It seems that the difference between these polecat species is accounted for the ecological pattern rather than the phylogenetic one. Resource partitioning and the lessening of their ecological niches' overlap in two sympatric carnivores could apparently explain the observed differences of their morphospaces. The morphological diversity of the European polecat is higher than that of the steppe polecat. A possible explanation of this phenomenon is likely to lie in the differences between prey ranges of these species. The morphological diversification in M. putorius could be facilitated by its adaptations to forest habitats of the temperate zone with a wide range of potential prey, whereas M. eversmanii could have evolved under more severe conditions of arid Eurasian habitats with a possible prey specialization.

Mitogenetic structure of brown bears (Ursus arctos L.) in northeastern Europe and a new time frame for the formation of European brown bear lineages.

We estimated the phylogenetic relationships of brown bear maternal haplotypes from countries of northeastern Europe (Estonia, Finland and European Russia), using sequences of mitochondrial DNA (mtDNA) control region of 231 bears. Twenty-five mtDNA haplotypes were identified. The brown bear population in northeastern Europe can be divided into three haplogroups: one with bears from all three countries, one with bears from Finland and Russia, and the third composed almost exclusively of bears from European Russia. Four haplotypes from Finland and European Russia matched exactly with haplotypes from Slovakia, suggesting the significance of the current territory of Slovakia in ancient demographic processes of brown bears. Based on the results of this study and those from the recent literature, we hypothesize that the West Carpathian Mountains have served either as one of the northernmost refuge areas or as an important movement corridor for brown bears of the Eastern lineage towards northern Europe during or after the last ice age. Bayesian analyses were performed to investigate the temporal framework of brown bear lineages in Europe. The molecular clock was calibrated using Beringian brown bear sequences derived from radiocarbon-dated ancient samples, and the estimated mutation rate was 29.8% (13.3%-47.6%) per million years. The whole European population and Western and Eastern lineages formed about 175,000, 70,000 and 25,000 years before present, respectively. Our approach to estimating the time frame of brown bear evolution demonstrates the importance of using an appropriate mutation rate, and this has implications for other studies of Pleistocene populations.